1. Field of the Invention
The present invention provides a novel granulation process using polymers with specific viscoelastic properties injected through one or more small holes into a moving bed of particles. More particularly, the invention relates to a process for making homogeneous fertilizer granules comprised of ureaform polymer and ammonium sulfate with reduction in the amount of recycle and possible elimination of a curing step.
2. Description of Related Art
Granulation refers to a process of agglomerating small particles by use of a liquid binder followed by drying or curing to make a solid larger particle, called a granule.
The fertilizer industry is a prime user of granulation processes. The most successful commercial fertilizer granulation process to date was developed by the Tennessee Valley Authority (TVA) and is a bulk granulation process. It uses a liquid binder prepared from ammonia and acids (see U.S. Pat. No. 3,153,574 and Hicks et al., xe2x80x9cBasis for Selection of Granulators at TVAxe2x80x9d, 28th Annual Meeting of the Fertilizer Industry Round Table, Oct. 31-Nov. 2, 1978).
The TVA method, and processes similar to it, are bulk granulation processes and are typical in fertilizer granulation: numerous granules are produced simultaneously and with a wide range of sizes. The wide size distribution necessitates the sieving of the product to obtain the desired granule size. The undersized and oversized granules are recycled, the oversized ones being crushed first. In many processes, the amount of recycle can be very high, often 3 to 4 times the feed rate (for instance, see U.S. Pat. Nos. 4,589,904 and 3,464,809). Such recycle has a significant adverse impact on costs because the equipment must be 3 to 4 times larger than necessary, and each pass through the process requires more binder, utilities, labor and time. In processes where there is significant recycle, much of the growth in average particle size comes from increasing the size of the individual particles by layering additional material on each discrete particle rather than agglomeration of several particles into one granule.
Ammonium sulfate has proven to be a difficult material to granulate (see for instance G. C. Hicks, et. al. in The Journal of Agriculture and Food Chemicals, Vol. 17, pages 306-311 (1969)). Hence, a number of improved processes have been developed in attempts to achieve economical and effective granulation of ammonium sulfate. For instance, U.S. Pat. No. 4,589,904 teaches the addition of metal sulfates, such as alum, to the typical ammonia-sulfuric acid binder to improve granulation of crystalline by-product ammonium sulfate. U.S. Pat. Nos. 3,725,029 and 4,743,289 teach the addition of lignosulfonates; however in contrast, U.S. Pat. No. 5,078,779 demonstrates the ineffectiveness of lignin sulfonate binders. These processes are disadvantageous because of cost, significant recycle, and/or poor granule properties.
Urea-formaldehyde, which has fertilizer capacity itself, has been employed as a polymeric binder to agglomerate particles. Exemplary teachings include U.S. Pat. Nos. 3,076,700, 3,705,794, 5,102,440, and 5,266,097 and DE 4,126,807. All of these ureaformaldehyde processes involve significant recycle, require expensive equipment and careful control, and generate products which require a drying step and/or suffer from dusting and dispensability problems.
In U.S. Pat. No. 4,554,004, the thermoplastic properties of urea phosphate are used as a binder. The resulting granules are soft and tacky, and require 12 to 16 hours of curing. U.S. Pat. No. 5,574,005 discloses an agglomeration process which uses a non-linear viscoelastic surfactant paste and a detergent builder in the agglomeration of detergent particles. The agglomerates are formed by treating the paste and builder in a high speed mixer/densifier then in a moderate speed mixer/densifier. This reference does not teach or suggest the use of fertilizer materials in this process, nor does it teach controlling the resulting granule size.
Processes which introduce a binder beneath the surface of a bed of fluidized particles are known. For instance, the layering process of U.S. Pat. No. 5,330,544, directed at achieving particles of uniform size, teaches spraying a saturated ammonium sulfate solution at the underside of a bed of ammonium sulfate feed crystals which are fluidized by a current of hot air. The resultant enlarged crystals, however, have a relatively high moisture content, requiring a drying process to reduce the moisture content to below 5%. This process does not teach a means of agglomerating smaller particles into a larger granule. U.S. Pat. No. 5,374,292, also a layering process, teaches the introduction of a sequence of reactive chemicals beneath the surface of a cascading bed of fertilizer particles to produce coated particles. A final coating of wax is introduced by similar means at the end of the process.
So it can be seen that there is a need for an improved granulation technology that: allows control of granule size, has low recycle and reduced environmental problems, exhibits overall cost-effective operation in today""s environment, permits formulations to be easily changed and to be multi-component, and can be used to readily granulate fine particles.
In response to the forgoing need in the art, I have developed an agglomeration process by which granules are formed by the steps of:
(a) injecting polymer having sufficient viscoelasticity through one of small holes or narrow tubes; and
(b) impacting particles on the polymer at the injection point.
The present invention is advantageous because it overcomes the problems of the old xe2x80x9cTVAxe2x80x9d ammoniation-bulk granulation process and subsequent art bulk processes. In addition to not being a bulk granulation process, other specific advantages of the instant invention include: low recycle and high throughput which result in low capital cost, control of granule size, ability to use ureaform polymers that are too viscous to spray, no odor or pollution problems, short drying or curing times, and low toxicity reagents. Ammonium sulfate, a common industrial by-product, is granulated efficiently and effectively without the need for sulfuric acid or liquid ammonia.
Other advantages of the present invention will be apparent from the following description, attached drawings, and attached claims.